1. Field of the Invention:
The present invention relates to a process for correction of the deformation of a picture formed by a cathode-ray tube and a device implementing this process. It relates more particularly to the adjusting of the S correction in a television set.
2. Description of the Related Art:
Cathode-ray tubes are presently the most commonly used display elements in television sets. Represented diagrammatically in FIG. 1a is a television 6 comprising a conventional cathode-ray tube. The latter comprises an electron gun 1 which produces an electron beam 2 which is accelerated and focused onto a luminescent screen 3 to form a point of the picture. Deflection coils 4, outside the tube, deflect the electron beam electromagnetically so as to scan the screen and form a complete picture.
Let us recall that a television picture is generally formed of two fields, even and odd, and that the electron beam scans the screen twice from top to bottom and from left to right in order to display a complete picture.
In what follows we shall be interested solely in the vertical deflection of the electron beam.
In FIG. 1c we have represented the current flowing through the coils 4 of the vertical deflector. The general shape of this signal is that of a sawtooth whose rising segments 8 correspond to the scanning of the screen by the electron beam from top to bottom (the active part of the picture), and whose segments 9 correspond to the return of the electron beam to the top of the screen (field blanking interval), the video signal sending a black level to the screen throughout the duration of the segment 9 (the inactive part of the picture). The duration of time between the start of a segment 8 and the end of a consecutive segment 9 is equal to the duration of a field.
Returning to FIG. 1a, it may be noted that the distance between the electron gun 1 and the surface of the screen 3 is not constant: it increases as we travel from the centre of the screen towards the top or the bottom. Indeed, in recent cathode-ray tubes termed "flat screens" the radius of curvature of the screen 3 onto which the electron beam 2 is projected is much greater than the radius of curvature R between the centre of deflection C of the electron beam and its theoretical surface of projection 5.
The consequences of this are that, when no correction is made, the picture which appears on the screen 3 is deformed, especially at the top and bottom of the picture. Thus, if a test pattern having equidistant horizontal lines is projected onto the screen, a picture such as that represented in FIG. 1b is obtained in which the gap between the lines is greater at the top and bottom of the picture than in the middle.
To correct this phenomenon, it is known practice to apply a current to the coils of the vertical deflector, not a perfect sawtooth current, as represented in FIG. 1c, but one in which the sawtooth exhibits "S"-shaped portions 18, as represented in FIG. 1e. This correction is dubbed "S correction". The image obtained when the correction is optimal is represented in FIG. 1d.
This S correction must be set at the end of the television production line by altering the amplitude of the "S" shape of the portions 18 relative to the uncorrected shape of the sawtooth 8'. During this adjustment, three points 10, 11, 12 of the portion 18 of the curve are not modified by the changes of amplitude of the S correction, these points will be called the fixed points of the S correction hereinafter.
In order for the correction of the picture to be optimal, it is also necessary for the S correction to be centred relative to the screen. That is to say, at the instants at which the sawtooth current passes respectively through the points 10, 11 and 12, the electron beam must scan the first line, the middle line and the last line of the screen respectively.
Moreover, in certain television receivers it is known practice to generate a few overscan lines at the top and bottom of the screen, these lines being used to perform tests. For example, these lines can be used to transmit colour information and to carry out auto-regulation of the colour temperature.
These overscan lines must not of course be displayed on the screen since they carry no information relating to the "useful" television picture transmitted. Hence, an adjustment must be made at the end of the television manufacturing line such that the electron beam does not scan the surface of the screen during these overscan lines.
Hence, at the end of a television receiver production line an operator must adjust a number of parameters: the position of the overscan lines, the position and amplitude of the S correction, etc.
In practice, he proceeds as follows:
firstly he dispatches a test pattern to the television, termed the convergence pattern and composed of a grid of equidistant lines, and he marks the zones of the picture which correspond to the fixed points of the S correction, that is to say he modifies the amplitude of the S correction so as to view on the screen those lines which remain still; PA1 once the position of these lines has been marked, he must secondly position them at the top, middle and bottom of the screen respectively by adjusting the amplitude and the offset of the sawtooth current flowing through the coils of the deflector; the purpose of this is to centre the fixed points of the S correction with respect to the picture formed on the screen and this makes it possible furthermore to avoid displaying the overscan lines on the screen; PA1 thirdly, he must readjust the amplitude of the S correction until the lines of the grid pattern appear equidistant on the screen. PA1 the generation of pulses whose duration is at least equal to a video line at the instants at which the curve of the current flowing through the deflection coils passes through points, termed the "fixed points" of the S correction, which are not affected by the variation in amplitude of said S correction; and PA1 the insertion of said pulses into the "Super Sand Castle" signal generated by the circuit controlling the scanning of the cathode-ray tube, said pulses indicating the position of those lines for which a black level is to be displayed on the screen. PA1 the generation of pulses whose duration is at least equal to a video line at the instants at which the curve of the current flowing through the deflection coils passes through points, termed the "fixed points" of the S correction, which are not affected by the variation in amplitude of said S correction; and PA1 the insertion of said pulses into the text insertion signal generated by a character generator so as to control the displaying of characters at the locations of the picture corresponding to said fixed points of the S correction. PA1 a sawtooth signal generator whose output is linked to the input of an S correction sinusoid generator, which also receives reference voltages as input and which outputs an S correction sinusoid which vanishes for input voltage values which are equal to the reference voltages, PA1 means for adding together the sawtooth signal delivered by the generator and said S correction sinusoid, PA1 means for comparing the sawtooth signal with each respectively of the reference voltages and for generating pulses at the instants at which the sawtooth signal passes respectively through each of the reference voltages; and PA1 means for displaying, as a function of the signal received containing said pulses, visual marks on the screen, at the location of the picture points which are not affected by the variation in amplitude of said S correction.